Potentially important food sources to primary consumers on shallow subtidal reefs include phytoplankton-dominated seston, kelp-derived detritus, and for locations adjacent to sources of freshwater runoff, terrestrially-derived POM. The SBCLTER is using stable carbon and nitrogen isotope ratio analysis of monthly water, kelp tissue and samples consumers of varying trophic status to evaluate the relative contribution of these sources to reef food webs. SBCLTER research has focused on beginning to characterize variability in the isotope values of potential food sources (phytoplankton, kelp, and terrestrial POM). To investigate sources of variability in values of marine POM, including contributions from phytoplankton, we are collecting monthly water samples and filtering them for POM from on and offshore of the reef, at locations less likely to be influenced by inputs from kelp or terrestrial runoff and more likely to reflect a primarily phytoplankton source at the three core SBCLTER research sites (Arroyo Quemado, Naples, Carpinteria). To identify the range of variability of isotopic ratio in kelp tissue, we concurrently collect kelp tissue samples from the three core research sites. To identify food sources used by reef consumers under different conditions of runoff, ocean climate and kelp production we have begun sampling a variety of reef consumers chosen to represent different trophic levels. Tissue samples were collected in April 2002 from the same species of consumers at four reef sites (Carpinteria, Naples, Mohawk, Arroyo Quemada), which vary in their proximity to sources of runoff and in their standing stock of giant kelp. We have also collected sediment samples at fixed distances from the source of runoff at four research sites (Naples, Goleta Bay, Mohawk, Carpinteria). This information will be used to evaluate whether these isotopic values differ enough from one another to permit the use of mixing models to estimate the contribution of each source to the reef food web.